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Well thats good Jack.....if you consider Clubs, Boiler Inspectors and Codes to be like the plague, then you have answered your own question .....
Some excellent designs for safety valves can be found if you Google Gordon Smith's designs for some published by Model Engineer magazine a number of years ago. I built a set to his design for my Bill Harris steam roller , they were easy to make, and they really work well. He specifies to use stainless steel springs from Lee Spring, one of the biggest commercial spring makers worldwide, so there is no guesswork about the springs themselves, and no corrosion. Lee Spring Co are alive and well as of Apr 2019. Head office is in Brooklyn NY. Pair of small springs for my valves from Lee was about $10 a few yr ago. Postage cost more than the springs!
The Sutton Coldfield ME club page has all the Gordon Smith safety valve drawings: https://www.scmes.co.uk/images/Technical%20Information/SafetyValves/Safety%20Valves%20Master.htmJust click on the list on the left to get the drawings.Jo